This invention relates to an electrophotographic photosensitive composition of a binder type which includes as a binder resin a water-soluble prepolymer capable of forming a network structure by cross-linking and as a photoconductive material a cadmium sulfide or similar inorganic compound (hereinlater referred to as "cadmium sulfide-group compound", which term is intended to mean semi-conductive compounds such as a sulfide and/or selenide of zinc or cadmium).
Photosensitive articles for electrophotography can be broadly classified into two categories: a metal plate vacuum-deposited with amorphous selenium; and a photosensitive article having a photosensitive layer obtained by coating one surface of a conductive substrate with a photosensitive composition obtained by dispersing a photoconductive inorganic compound in a solution of a binder resin in an organic solvent. The present invention more specifically relates to an electrophotographic photosensitive composition (hereinafter referred to briefly as photosensitive composition) suitable for the fabrication of the latter binder type photosensitive article for electrophotography.
Known photosensitive compositions are generally prepared by dispersing photoconductive inorganic compounds in solutions which are obtained by dissolving homopolymers or interpolymers of monomers such as ethylene, styrene, methacrylic acid esters, acrylic acid esters, vinyl acetate, vinyl chloride, vinylidene chloride, butadiene, etc., silicon resin, epoxy resin, or alkyd resin, in organic solvents such as acetone, benzene, toluene, ethyl acetate, trichloroethylene and the like. A cadmium sulfide-group compound which is one of the photoconductive inorganic compounds has inherently a greater affinity for water than for organic solvents and is thus hard to disperse in solutions of resins in organic solvents. This makes it very difficult to obtain a uniform dispersion by dispersing the cadmium sulfide-group compound in the binder solutions in organic solvents even if a great deal of expense and labor is employed. That is, in known photosensitive compositions using organic solvents to prepare resin solutions, the resins dissolved in organic solvents are generally poor in affinity for and adsorptivity on the powder of a photoconductive cadmium sulfide-group compound, so that not only the powder of the photoconductive cadmium sulfide-group compound can not be covered satisfactorily with the resin, but also organic solvent remaining between the powder of the photoconductive cadmium sulfide-group compound and the resin is evaporated when the photosensitive composition is applied onto a conductive substrate and dried, leaving voids in the resulting photosensitive layer. These voids increase hygroscopicity of the photosensitive layer, lowering the electrophotographic properties of the layer.
On the other hand, in order to improve the dispersiveness of photoconductive inorganic compounds and avoid ill effects of organic solvents the use of which is generally unfavorable from a viewpoint of environmental pollution or public nuisance, there are known photosensitive compositions which are prepared by dispersing photoconductive inorganic compounds in aqueous resin dispersions or resin latices. However, these aqueous photosensitive compositions are disadvantageous in that the photosensitive layer derived from such aqueous composition tends to inevitably contain moisture therein and even a trace of the moisture in the photosensitive layer adversely affects the electrophotographic properties of the layer to a considerable extent. For instance, photosensitive compositions obtained by dispersing powder of photoconductive cadmium sulfide-group compounds in aqueous latices fail to produce a photosensitive layer which is excellent in moisture proofness since voids are likely to be formed in the photosensitive layer obtained from such composition when the composition is applied onto a conductive substrate and dried to form a film from the latex. Accordingly, aqueous photosensitive compositions are not yet of practical use.